A need exists for a wide band analog signal processing device having a bandwidth in the order of 1 gigahertz (GHz) or greater. Applications for these devices exist in the fields of radar, electronic warfare and transient recording. As the bandwidth increases beyond the 1 GHz range, it becomes necessary to utilize devices other than simple electronic components. In this respect, a device of excellent promise called an optoelectronic switch has been developed. In an optoelectronic switch, an optical pulse such as may be provided from a laser is used to turn on and off an electrical signal. Such a switch has a number of advantages over electronic switches, the most significant being the complete isolation provided between the switching command signal from the laser and the analog signal which is being switched. Incomplete isolation can be a severe source of signal distortion. Also, with an optoelectronic switch, it is possible to take advantage of the short, high repetition rate pulses from mode-locked lasers to perform electronic waveform switching, detection and modulation at rates and resolution far exceeding those obtainable in an all electronic circuit.
An early optoelectronic switch is described in Applied Physics Letters, Vol. 26, No. 3 (Feb. 1, 1975) in an article by D. H. Auston of Bell Laboratories entitled "Picosecond Optoelectronic Switching and Gating in Silicon." The Auston device consists of a thin slab of high resistivity (10.sup.4 ohm-cm) silicon on which a microstrip transmission line was fabricated. The microstrip line consisted of a uniform aluminum ground plane on the bottom and a narrow aluminum strip for an upper conductor. The upper conductor had a gap of high resistance which in the "Off" state prevented the transmission of a signal across the device. A 0.53 micrometer (.mu.m) pulse from a mode-locked neodymium glass laser was projected on the gap and produced a thin high conductivity region near the top surface of the silicon crystal. This turned on the switch to the "On" state permitting the signal to be transmitted across the circuit. A second optical pulse of 1.06 .mu.m was used to close the gate. This 1.06 .mu.m pulse, because of its wave length, was able to penetrate the crystal to the ground plane shorting the transmission line and preventing further transmission by totally reflecting the incident electrical wave. The early optoelectronic switch of Auston was improved by Chi H. Lee as reported in Applied Physics Letters, Vol. 30, No. 2 (Jan. 15, 1977) in an article entitled "Picosecond Optoelectronic Switching in GaAs." In the Lee device, the silicon substrate was replaced by a Gallium Arsenide (GaAs) slab mounted on an alumina insulator. The gallium arsenide was chromium (Cr) doped to make the gallium arsenide semi-insulating. With the doping, the slab had a resistivity of 1.times.10.sup.6 ohms-cm. Since the carrier lifetime in this type of gallium arsenide is less than 100 picoseconds, Lee found that the GaAs optoelectronic switch did not require an optical pulse to switch it "off" as did the silicon device of Auston.
Both the Si and GaAs devices have generally been fabricated in geometries that required a high peak laser power and a very low-repetition-rate laser. For high speed signal processing, high repetition rate laser pulses are required and the laser will have relatively low peak power. The fast response of the GaAs switch makes it the more attractive device for high speed applications. However, attempts to fabricate efficient GaAs switches and use them with high repetitive rate lasers have led to disappointingly high on-state impedance (500 ohms-1000 ohms). While the reason for this is not fully understood, it is believed to be associated with fundamental GaAs materials properties (e.g., high surface recombination velocity). Accordingly, a need still exists for an optoelectronic switch which, in conjunction with a high repetition rate laser, has a very fast response time on the order of 50 picoseconds or less and a low on-state impedance on the order of 50 ohms or less. In this connection, it should be noted that on-state impedance is critical in a switch inasmuch as the lower the on-state impedance, the greater the sensitivity of the switch.